More effort should be directed towards preventing the uptake of cannabis by young people argue US researchers, after a study of more than 1,000 New Zealanders found it could cause memory loss and drops in IQ scores by mid-adulthood.

The study, which followed the participants until age 38, found those that started using cannabis in adolescence and continued to use it experienced a greater IQ decline compared with adult-onset users.

In their paper, published in PNAS, Madeline Meir and colleagues write that quitting or reducing cannabis use did not fully restore neuropsychological functioning among that same group, suggesting that taking up cannabis in adolescence when the brain is undergoing critical development, could have neurotoxic effects.

“Adolescence is a particularly critical period of brain development and maturation,” said Nadia Solowij, Associate Professor and ARC Future Fellow in the School of Psychology at University of Wollongong.

“As such, it may be more vulnerable to insult from drugs than the more mature adult brain.”

While evidence has been accumulating for impaired function in both adult and adolescent samples, Professor Solowij said evidence for ongoing impairment after cessation of use has been mixed.

“This study was able to rule out a range of potential confounds that are often suggested to underlie reports of adverse effects on cognitive function in cannabis users.”

The study’s findings add to the case for preventive public health education to reduce adolescent initiation and use of cannabis, said Professor Wayne Hall, NHMRC Australia fellow deputy director at The University of Queensland Centre for Clinical Research.

Professor Hall said in the past it has been difficult to decide whether cannabis was the cause of cognitive impairment, or cognitively impaired young people were more likely to become involved in regular cannabis use.

“This prospective study greatly strengthens the case for regular cannabis use being a cause of cognitive decline by mid adulthood in young people.”

Robin Murray, Professor of Psychiatric Research, at, Kings College in London, said we have known for some time that heavy use of cannabis increases risk of schizophrenia-like psychoses.

“There are far fewer studies on its effect on minor psychiatric illness or on everyday life. However, there are a lot of clinical and educational anecdotal reports that cannabis users tend to be less successful in their educational achievement, marriages and occupations…This study provides one explanation as to why this might be the case.”